Various tests are available that can be used to assess the cleanliness of a surface. Such tests include those based on the detection of ATP using the firefly luciferase reaction, tests based on the detection of protein using colorimetry, tests based on the detection of micro-organisms using microbiological culture techniques, and tests based on detection of micro-organisms using immunochemical techniques. Surfaces can be sampled using either a swab device, or by direct contact with an agar medium.
Although the above tests are useful in the detection of a contaminated surface, they are typically prone to errors. These errors may be due to the performance of the measurement apparatus (a light detection device, such as a luminometer, colour detection by human assessor, or test result interpretation by human assessor).
It is also believed that errors may be due to variations in the assay conditions, for example being caused by assay temperature. Errors may also be due to the detection “reagents” used in the test systems, including firefly luciferase reagents for ATP, colorimetric reagents for protein detection, microbiological media for bacteriological analysis etc. For example, abused or badly stored reagents can give rise to false-positive or false-negative results.
Errors may also arise due to differences in sampling procedure. For example, “swabs” are often used for sample collection in such hygiene assays. These can give rise to variation due to the presence or absence of extraction agents and/or surface active components.
Furthermore, errors can arise due to differences in operator technique during the performance of the assay itself. Although this may be due to an error on the part of the operator, it is more common that the variance in performance is a result of subtle differences in technique.
All of the above sources of potential error ultimately lead to a reduction in confidence in the validity of such test results. In the case of safety critical applications there are significant consequences if a set of invalid results are generated.
There are a number of different methods for dealing with the inherent variability of surface hygiene tests. Such methods include:
1. Use of chemical standards (such as the use of solutions of ATP to calibrate ATP tests) (Jago, P H, Stanfield, G, Simpson, W J & Hammond, J R M 1989. In ATP Bioluminescence: Rapid Methods in Microbiology, Society of Applied Microbiology Technical Series, Vol. 26, Stanley P E et al [eds] pp 53-61). These can be applied in two ways. In the external calibration technique, the test response is compared to that obtained in the presence of known concentrations of the analyte. In the internal calibration technique, a known amount of analyte is added to the test after measurement of the sample signal. The ratio of the signal from the test sample to that of the standard can be used to calculate the amount of analyte present in the sample.
2. Use of a light sensitive derivative of the analyte to assure test performance. This is the basis of the PhotoQuant® technique for assay calibration (Method for calibrating chemical assays, PCT/GB95/00794);
3. Use of low level light emitting devices to assure the performance of instrumentation, or use of coloured cards to act as an aid to the judgment (by eye) of calorimetric tests etc. For example, radioactive material employing a scintillant are employed in the Biolink light standards (Leaback, D H, Easy-to-use light standards as aids to luminometry, Szalay, AA et al [Eds], pp 33-37, Bioluminescence of the VII International Symposium on Bioluminescence and Chemiluminescence, John Wiley & Son, Chichester 1993).
Whilst these methods are useful in some circumstances, there are a number of limitations. All of the approaches identified above are limited by the fact that they do not respond to differences in operator swab technique. Faulty sampling techniques (which may include failure to swab the required area, or failure to apply sufficient pressure during the swabbing process) can lead to low results (or even negative results).